The invention relates to a process for adjusting the structure and/or surface characteristics of oxide materials.
In order to make oxide materials usable in processes in which the catalytic and/or adsorptive properties of these materials are important, it is necessary to be able to controllably and reproducibly adjust their structural and/or surface characteristics. According to the conventional state of the art, this adjustment is accomplished by thermal treatment of inorganic oxide gels, whereby a formed or unformed gel is dried at elevated temperature and subsequently tempered at an even higher temperature. The surface characteristics can thereby be adjusted, on the one hand, by special measures during the production of the oxide gel such as inclusion of additives, adjustment of the pH value, etc., and on the other hand by varying the time and temperature profiles of the required thermal treatment.
It is also known to remove the solvent from inorganic gels by thermal treatment under supercritical conditions. Thus, in U.S. Pat. No. 2,093,454, a process is described in which an aqueous silica gel is raised in an autoclave to a temperature which lies above the critical temperature of the water (+374.degree. C.). The water is withdrawn from the autoclave under these conditions, and a dry gel is obtained. Due to the high solvating power of the water, the originally formed silica gel may be dissolved again before reaching the critical temperature and then, after drying, appears as a finely divided powder. In addition, this process requires very high temperatures. In order to produce oxide gels in which the structure of the solvent-containing gel will remain intact, it is therefore preferred in this process to first displace the water with a solvent which has a lower critical temperature than water and then to evaporate this solvent at corresponding temperatures. This process is also very expensive because a further process step is necessary, because costly solvents must be utilized, and because substantially elevated temperatures are still required.